Well let's not forget that most seashells begin life as swimmers.  The
larval mollusk that hatches from the egg (in many families) is a
free-swimming organism, and only settles down to a more sedentary
existence after it become weighed down by that newly developed
calcium carbonate anchor we call a shell.  (Not all mollusks follow
this pattern though - in some the larval form passes within the egg,
and a shelled minature of the adult hatches out.)  As for mollusks
which continue swimming when mature, not many shelled species do so,
for the obvious reason that the weight of a shell is
counter-productive if your purpose is to remain suspended in the
water.  Swimmers that have a substantial shell either restrict their
swimming to short bursts in emergency situations (Pecten, Lima) or
they have a method of compensating for the weight of the shell (the
gas-filled buoyancy chambers in Nautilus).  The best molluscan
swimmers though, have eliminated the shell altogether, or have moved
it to an internal location.  Of course, in avoiding the weight of a
shell, they have also forfeited the protection which a hard shell
provides, and therefore rely on speed or other special adaptations
for protection.  The best swimmers among the Mollusca are the
Cephalopods.  They can all handle themselves pretty well in open
water, whether shell-less (Octopus), externally shelled (Nautilus),
Internally shelled (Spirula), externally pseudo-shelled (Argonauta)
or internally pseudo-shelled (Loligo and other squids).  Among the
bivalves, the Pectinidae (scallops or pectens) and Limidae (file
shells) are the best swimmers, but they must maintain a high level of
activity to stay up in the water.  As soon as they rest, they sink to
the bottom.  Some species of Solenidae (razor clams and jackknife
clams), such as Ensis, sometimes swim "accidentally".  The rapid
motions of the large foot are actually an effort to dig into the
sand, but if the foot isn't in contact with the sand, it pulls them
through the water in a crazy zigzag manner, until they find the
substrate, and rapidly dig into it.  Among the gastropods, I believe
the best swimmers are the Aplysiidae or "sea hares", a family with an
odd fan-shaped internal shell.  Some of them can easily outdistance a
diver.  Some of the nudibrachs (sea slugs) are free swimming, and so
are the pelagic Pteropods (sometimes called butterfly snails), in
which the edges of the foot are expanded into two wide flaps
resembling wings.  These do have a shell, but it is extremely thin
and lightweight.
Paul M.
Rhode Island